In a parallel system composed of multiple parallel-connected modules (e.g. inverters), AC outputs from respective inverters are connected in parallel for jointly providing energy for a load so as to improve the capacity or reliability of the system. To ensure reliable operation of the parallel inverter system, the amplitudes of the respective inverters should be the same and the phases should be synchronized. Otherwise, high circulating current may occur between the parallel-connected inverters, thereby resulting in overload or damage of the inverters.
In a decentralized parallel inverter system, in order to ensure the phase synchronization among the respective inverters, one of the inverters needs to be set as a master unit while the others act as slave units. The master unit can act as the frequency source for the output voltage of the entire parallel inverter system, and the slave units track the phase of the output voltage of the master unit. For instance, when the bypass power source fails or goes abnormal in a parallel UPS system, every inverter loses the common tracking source. This requires adopting a control strategy of determining a master and slave units, i.e., establishing only one master unit to maintain the phase synchronization of all the inverters.
In such a decentralized controlling strategy, it requires one and only one master unit employed in the system. The reason is as follows: if no master unit exists in the parallel inverter system, each inverter can track the output phase of the system and maintain substantially the phase synchronization of the inverters. However, the entire parallel inverter system is in the state of self-excited oscillation such that the ultimate output frequency of the system will be diverted from the nominal value. If multiple master units exist in the parallel inverter system, each master unit serves as the frequency source of the system and generates output voltage according to the respective nominal values. Since dispersion exists inevitably between the respective nominal frequency oscillators a significant phase difference will occur over time in the output voltage of the respective inverters no matter if the initial state is the same or not, thereby leading to the failure of the parallel connection. In addition, the master unit herein may also be used for time sequential controls in the parallel inverter system.
In the aforesaid phase synchronization strategy, in order to ensure the existence and uniqueness of the master unit, a number of methods are provided as follows:    (1) The master unit is set manually. A disadvantage of this method is that it is not flexible. The manually set inverter has to be turned on before the parallel inverter system operates. While a fault occurs in the inverter, the parallel inverter system composed of the rest inverters cannot run properly before a new master unit is set.    (2) As illustrated in FIG. 1, the master unit determination relies on the netlike parallel signal cables laid among the inverters. Any of the inverters can detect and identify the state of other inverters. However, a disadvantage of this method is that, due to the netlike parallel signal connection among the inverters, the connection cables become complicated when large number of inverters is used.    (3) As illustrated in FIG. 2, the master unit determination relies on the the serial communication among the inverters. The communication line used between the inverters may be RS485, CAN bus, etc. However, a drawback of this method lies in the unsatisfied real time capability and reliability.            In addition to parallel inverter system, the parallel rectifier system and some master-slave communication system such as RS485 network require establishing the master unit as well.        